Method for preparing nickel nanoparticles

ABSTRACT

Provided is a method for preparing nickel nanoparticles capable of easily controlling particle sizes and shapes of the nickel nanoparticles and obtaining a high yield of the nickel nanoparticles using a process that is simpler than methods used to mass-produce the nickel nanoparticles. The method for preparing nickel nanoparticles may be useful to prepare nickel nanoparticles by mixing a nickel precursor and organic amine to prepare a mixture and heating the mixture.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No. 2008-76448 filed on Aug. 5, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing nickel nanoparticles, and more particularly, to a method for preparing nickel nanoparticles capable of easily controlling particle sizes and shapes of the nickel nanoparticles and obtaining a high yield of nickel nanoparticles using a process that is simpler than methods used to mass-produce the nickel nanoparticles.

2. Description of the Related Art

Nickel may be used as an electrode material, or a catalyst of a fuel cell, a catalyst in hydrogenation or a catalyst in various chemical reactions in the field of a variety of applications. For example, nickel has been used as an internal electrode material of a multi-layer ceramic capacitor (MLCC), or a material to improve loading density. Also, nickel has been used as a catalyst in the field of a fuel cell and organic synthesis, and there have been ardent attempts to develop nickel particles as an alternative to noble metals such as platinum. With the recent tendency toward thin, small and high-capacity multi-layer ceramic capacitor, there have been attempts to reduce the size of the nickel particles used inside the multi-layer ceramic capacitor, and there is also an attempt to prepare nanosized nickel particles.

Nickel nanoparticles may be prepared using various methods such as liquid phase deposition, vapor phase deposition, plasma and laser. Among the various methods, methods for preparing nickel nanoparticles in a liquid phase have developed to reduce the manufacturing cost.

Among the methods for preparing nickel nanoparticles in a liquid phase, there is a method for preparing nickel particles by adding sodium hydroxide to a mixture solution of nickel chloride hydrate and hydrazine as a reducing agent (Choi. J.-Y. et al, J. Am. Ceram. Soc. 2005, vol. 88, p. 3020). The method includes: reacting hydrazine with nickel chloride to form a complex compound and adding sodium hydroxide to the complex compound to form nickel particles. In particular, it is possible to adjust the size of the nickel particles to a size of 87 to 203 nm according to the ratio of nickel chloride/hydrazine/sodium hydroxide. However, the nickel particles prepared according to the method have problems in that it is difficult to disperse the nickel particles since they are necked to each other, and surfaces of the nickel particles are also not smooth but rough.

Also among the methods for preparing nickel particles in a liquid phase using hydrazine as the reducing agent, there is a method for controlling the size of nickel particles by adding a trace of cobalt (Kim, K.-M. et al, J. Electroceram. 2006, vol. 17, p. 339.). In this method, nickel chloride or acetic acid nickel hydrate was used as the nickel precursor. Here, the nickel particles were prepared by mixing hydrazine with a nickel precursor to form a mixture solution and adding sodium hydroxide to the mixture solution. In this case, it is possible to control the size of the nickel particles by adding a trace of cobalt chloride to the mixture solution of nickel precursor and hydrazine. The nickel particles prepared according to the method are in a range of 150 to 450 nm, and reduced in size with an increasing content of the added cobalt. The addition of cobalt causes an increase in the number of formed nucleus to control the size of the nickel particles, but rough surfaces of the resulting nickel particles and their necking behavior are still similar to those of the nickel particles prepared by the previous methods.

As an alternative to control the size of nickel particles under the control of nucleation in the art, there is a method for preparing nickel particles by adding palladium or silver ions, which facilitate the nucleation, to a solution including a nickel precursor and a surfactant and incorporating reducing agents hydrazine and ammonia (Chou, K.-S. et al, J. Nanoparticle Res. 2001, vol. 3, p. 127.). The size of the nickel particles prepared according to this method is in a range of 10 to 25 nm, which is significantly reduced, compared to the convention methods. However, the synthesized nickel particles have problems in that some nickel hydroxide, in addition to pure nickel, is present in the nickel particles, and it is impossible to mass-produce the nickel particles due to the very low reaction concentration.

In addition to the method for preparing nickel particles using the nickel precursor and the reducing agent hydrazine, there is also a known method for preparing nickel particles by thermally cracking a nickel alkoxide precursor. In this method, the nickel particles are prepared by synthesizing a nickel-aminoalkoxy metal complex compound, dissolving the complex compound in an organic solvent such as toluene and heating the resulting mixture to thermally crack the complex compound. Here, synthesized nickel particles is very small in size with diameters ranging from 3 to 5 nm, but the nickel particles having various shapes such as rod as well as the spherical shape are present in a mixed form, and also entangled with each other. This preparation method has problems in that an additional process of preparing a metal complex compound is required, it is difficult to mass-produce the metal complex compound, and the nickel particles are too small in size to use the metal complex compound as an internal electrode material of the multi-layer ceramic capacitor.

Therefore, there is a continuous demand for developing methods that may more easily control the sizes and shapes of the nickel nanoparticles, as well as mass-producing the nickel nanoparticles with lower expense.

SUMMARY OF THE INVENTION

The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a method for preparing nickel nanoparticles capable of easily controlling particle sizes and shapes of nickel nanoparticles and obtaining a high yield of nickel nanoparticles using a process that is simpler than methods used to mass-produce the nickel nanoparticles.

According to an aspect of the present invention, there is provided a method for preparing nickel nanoparticles including: mixing a nickel precursor, organic amine and a reducing agent to prepare a mixture; and heating the mixture. Here, an organic solvent may be further mixed with the mixture.

In this case, the nickel precursor may be at least one selected from the group consisting of nickel chloride (NiCl₂), nickel sulfate (NiSO₄), nickel acetate (Ni(OCOCH₃)₂), nickel acetylacetonate (Ni(C₅H₇O₂)₂), halogenated nickel (NiX₂, wherein X is F, Br, or I), nickel carbonate (NiCO₃), nickel cyclohexanebutyrate ([C₆H₁₁(CH₂)₃CO₂]₂Ni), nickel nitrate (Ni(NO₃)₂), nickel oxalate (NiC₂O₄), nickel stearate (Ni(H₃C(CH₂)₁₆CO₂)₂), nickel octanoate ([CH₃(CH₂)₆CO₂]₂Ni) and hydrates thereof.

Also, the organic amine may be presented by C_(n)NH₂ (wherein, n is an integer of 4≦n≦30). For example, the organic amine may include at least one selected from the group consisting of oleyl amine, dodecyl amine, lauryl amine, octyl amine, trioctyl amine, dioctyl amine and hexadecyl amine.

In addition, the reducing agent may include, for example, at least one selected from the group consisting of sodium borohydride (NaBH₄), tetrabutylammonium borohydride ((CH₃CH₂CH₂CH₂)₄N(BH₄)), lithium aluminumhydride (LiAlH₄), sodium hydride (NaH), borane-dimethylamine complex((CH₃)₂NH.BH₃) and alkanediol (HO(CH₂)_(n)OH, wherein n is an integer of 5≦n≦30).

Additionally, the organic solvent may include at least one selected from the group consisting of an ether-based organic solvent (C_(n)OC_(n), wherein n is an integer of 4≦n≦30), a saturated hydrocarbon-based organic solvent (C_(n)H_(2n+2), wherein n is an integer of 7≦n≦30), an unsaturated hydrocarbon-based organic solvent (C_(n)H_(2n), wherein n is an integer of 7≦n≦30), and an organic acid-based organic solvent (C_(n)COOH, wherein n is an integer of 5≦n≦30). For example, the ether-based organic solvent may be at least one selected from the group consisting of trioctylphosphine oxide, alkyl phosphine, octyl ether, benzyl ether, and phenyl ether, and the saturated hydrocarbon-based organic solvent may be at least one selected from the group consisting of hexadecane, heptadecane and octadecane. Also, the unsaturated hydrocarbon-based organic solvent may be at least one selected from the group consisting of octene, heptadecene and octadecene, and the organic acid-based organic solvent may be at least one selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid and hexadecanoic acid.

In the operation of heating the mixture, the mixture may be heated to a temperature of 50 to 450° C., and the heating time may be in a range from 1 minute to 8 hours.

Furthermore, the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention may further include: separating the nickel nanoparticles from the heated mixture after the operation of heating the mixture. Here, the operation of separating the nickel nanoparticles from the heated mixture may include: adding ethanol or acetone to the heated mixture to precipitate the nickel nanoparticles and separating the precipitated nickel nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 1 are observed with a transmission electron microscope.

FIG. 2 is a diagram illustrating a particle size distribution of the nickel nanoparticles.

FIG. 3 is a diagram illustrating an electron diffraction result of the nickel nanoparticles.

FIG. 4 is a diagram illustrating an analytic result of X-ray diffraction pattern of the nickel nanoparticles.

FIG. 5 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 2 are observed with a transmission electron microscope.

FIG. 6 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 3 are observed with a transmission electron microscope.

FIG. 7 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 4 are observed with a transmission electron microscope.

FIG. 8 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 5 are observed with a transmission electron microscope.

FIG. 9 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 6 are observed with a transmission electron microscope.

FIG. 10 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 7 are observed with a transmission electron microscope.

FIG. 11 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 8 are observed with a transmission electron microscope.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, it is apparent to those skilled in the art that modifications and variations may be made without departing from the scope of the invention. Therefore, the exemplary embodiments of the present invention will be provided for the purpose of better understanding of the present invention as apparent to those skilled in the art.

In the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention, first of all, the nickel nanoparticles are prepared by mixing a nickel precursor and organic amine to prepare a nickel precursor mixture and heating the nickel precursor mixture to thermally crack the nickel precursor mixture.

The nickel precursor, which may be used in the present invention, may be at least one selected from the group consisting of, but is not particularly limited to, nickel chloride (NiCl₂), nickel sulfate (NiSO₄), nickel acetate (Ni(OCOCH₃)₂), nickel acetylacetonate (Ni(C₅H₇O₂)₂), halogenated nickel (NiX₂, wherein, X is F, Br, or I), nickel carbonate (NiCO₃), nickel cyclohexanebutyrate ([C₆H₁₁(CH₂)₃CO₂]₂Ni), nickel nitrate (Ni(NO₃)₂), nickel oxalate (NiC₂O₄), nickel stearate (Ni(H₃C(CH₂)₁₆CO₂)₂), nickel octanoate ([CH₃(CH₂)₆CO₂]₂Ni) and hydrates thereof. Here, any of compounds that may be used as a nickel source may be used as the nickel precursor in the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention.

Unlike the conventional methods for preparing nickel nanoparticles, organic amine is used in the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention. The organic amine may function as an organic solvent, or a reducing agent. When an additional solvent is added to the mixture of nickel nanoparticles, an organic solvent may be used instead of an aqueous solvent due to the added organic amine.

In accordance with one exemplary embodiment of the present invention, since the organic amine is used to prepare the nickel nanoparticles, the prepared nickel nanoparticles may be coated with the organic amine. Therefore, the nickel nanoparticles have excellent dispersing property with respect to other organic solvents when the nickel nanoparticles will be used later. Therefore, when the nickel nanoparticles are, for example, dispersed in the organic solvent so as to apply the nickel nanoparticles to an internal electrode of a multi-layer ceramic capacitor, an additional process is not required due to the dispersion of the nickel nanoparticles.

The organic amine may be represented by C_(n)NH₂ (wherein, n is an integer of 4≦n≦30). The organic amine that may be used in the present invention, for example, includes, but is not particularly limited to, oleyl amine, dodecyl amine, lauryl amine, octyl amine, trioctyl amine, dioctyl amine and hexadecyl amine.

In addition to the organic amine, the nickel precursor mixture may further include an organic solvent when an additional solvent is used in the nickel precursor mixture.

The organic solvent used herein includes at least one selected from the group consisting of, but is not particularly limited to, an ether-based organic solvent (C_(n)OC_(n), wherein n is an integer of 4≦n≦30), a saturated hydrocarbon-based organic solvent (C_(n)H_(2n+2), wherein n is an integer of 7≦n≦30), an unsaturated hydrocarbon-based organic solvent (C_(n)H_(2n), wherein n is an integer of 7≦n≦30) and an organic acid-based organic solvent (C_(n)COOH, wherein n is an integer of 5≦n≦30).

The ether-based organic solvent that may be used in the present invention, for example, includes, but is not particularly limited to, trioctylphosphine oxide (TOPO), alkyl phosphine, octyl ether, benzyl ether and phenyl ether.

The saturated hydrocarbon-based organic solvent that may be used in the present invention, for example, includes, but is not particularly limited to, hexadecane, heptadecane and octadecane. Also, the unsaturated hydrocarbon-based organic solvent that may be used in the present invention includes, but is not particularly limited to, octene, heptadecene and octadecene.

The organic acid-based organic solvent that may be used in the present invention includes, but is not particularly limited to, oleic acid, lauric acid, stearic acid, mysteric acid and hexadecanoic acid.

A reducing agent is mixed with the nickel precursor mixture. The reducing agent that may be used in the present invention, for example, includes, but is not particularly limited to, sodium borohydride (NaBH₄), tetrabutylammonium borohydride (TBAB, (CH₃CH₂CH₂CH₂)₄N(BH₄)), lithium aluminumhydride (LiAlH₄), sodium hydride (NaH), borane-dimethylamine complex((CH₃)₂NH.BH₃) and alkanediol (HO(CH₂)_(n)OH, wherein n is an integer of 5≦n≦30).

The nickel precursor mixture is heated and thermally cracked. A heating temperature of the nickel precursor mixture may be in a range of 50 to 450° C., preferably 60 to 400° C., and more preferably 80 to 350° C. And the nickel precursor mixture may be heated for 1 minute to 8 hours.

When the nickel precursor mixture is heated and thermally cracked in the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention, the nickel nanoparticles are prepared. The prepared nickel nanoparticles may be separated from the nickel precursor mixture, for example, by adding ethanol or acetone to the heated nickel precursor mixture to precipitate the nickel nanoparticles and centrifuging the precipitated nickel nanoparticles.

In accordance with one exemplary embodiment of the present invention, sizes of the prepared nickel nanoparticles may be controlled more effectively according to the reaction conditions. In the following Examples, nickel nanoparticles are prepared by changing concentration of a nickel precursor, concentration and kinds of reducing agents and/or reaction temperature to control the sizes of the nickel nanoparticles.

EXAMPLES

Hereinafter, exemplary embodiments of the present invention are described in more detail. In the following Examples 1 to 8, nickel nanoparticles are prepared according to the method of the present invention.

Example 1

Preparation of Nickel Nanoparticles

13 g of nickel chloride as a nickel precursor, 200 ml of oleyl amine as organic amine, and 0.5 g of tetrabutylammonium borohydride (TBAB) as a reducing agent were put into a flask under an argon atmosphere, mixed, and then heated to a temperature of 100° C. The resulting mixture solution was kept at the temperature for 1 hour. The mixture solution was heated to a temperature of 160° C., and kept for 1 hour. Then, the flask is cooled to a room temperature, and 300 ml of ethanol was added to the mixture solution to precipitate nanoparticles, and 6.1 g of the precipitated nanoparticles were recovered using a centrifuge. In this case, a reaction yield of the nanoparticles was 99% or more.

10 mg of the synthesized nickel nanoparticles were dispersed in a solvent such as alcohol or toluene. 20 μl of a nickel nanoparticle-containing solution was dropped onto a TEM grid (commercially available from Ted Pella Inc.) coated with carbon film, dried for approximately 20 minutes, and then observed using a transmission electron microscope (HRTEM, commercially available from Philips, acceleration voltage: 200 kV). FIG. 1 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 1 are observed with a transmission electron microscope. Referring to FIG. 1, it was revealed that the nickel nanoparticles prepared in Example 1 have a uniform size and a round particle shape. The size of the nickel nanoparticles observed with the transmission electron microscope were measured, and a size distribution of the nickel nanoparticles was shown in FIG. 2. Here, an average particle size of the nickel nanoparticles was 50.8±10 nm.

Also, a crystal structure of the nickel nanoparticles was observed using an electron diffraction in the transmission electron microscope. FIG. 3 is a diagram illustrating an electron diffraction result of the nickel nanoparticles prepared in Example 1. From the electron diffraction result, it was confirmed that the prepared nickel nanoparticles have a cubic crystal structure. In addition, an X-ray diffractometer (commercially available from Rikagu) was used to analyze the crystal structure of the nickel nanoparticles. FIG. 4 is a diagram illustrating an analytic result of X-ray diffraction pattern of the nickel nanoparticles prepared in Example 1. From the analytic result of X-ray diffraction pattern, it was also confirmed that the nickel nanoparticles have the same cubic crystal structure as in the electron diffraction result.

Example 2

Nickel nanoparticles were prepared in the same manner as in Example 1, except that 6.5 g of nickel chloride was used to control an amount of the nickel precursor. FIG. 5 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 2 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles have a size of 38.3±11 nm, which is smaller than the nickel nanoparticles prepared in Example 1.

Example 3

Nickel nanoparticles were prepared in the same manner as in Example 1, except that 26 g of nickel chloride was used to control an amount of the nickel precursor. FIG. 6 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 3 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles have a size of 94±22 nm, which is larger than the nickel nanoparticles prepared in Example 1.

Example 4

Nickel nanoparticles were prepared in the same manner as in Example 1, except that the nickel nanoparticles were prepared at 180° C. so as to control a reaction temperature. FIG. 7 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 4 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles are larger in size than the nickel nanoparticles prepared in Example 1.

Example 5

Nickel nanoparticles were prepared in the same manner as in Example 1, except that the nickel nanoparticles were prepared at 200° C. so as to control a reaction temperature. FIG. 8 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 5 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles are larger in size than the nickel nanoparticles prepared in Example 1.

Example 6

Nickel nanoparticles were prepared in the same manner as in Example 1, except that TBAB was not used to control an amount of the added reducing agent. FIG. 9 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 6 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles are larger in size than the nickel nanoparticles prepared in Example 1, and also that the nickel nanoparticles with other shapes in addition to the round shape are prepared.

Example 7

Nickel nanoparticles were prepared in the same manner as in Example 1, except that 0.25 g of TBAB was used to control an amount of the added reducing agent. FIG. 10 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 7 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles are larger in size than the nickel nanoparticles prepared in Example 1.

Example 8

Nickel nanoparticles were prepared in the same manner as in Example 1, except that 2.6 g of 1,2-hexadecanediol was used instead of TBAB. FIG. 11 is a diagram illustrating a result obtained when the nickel nanoparticles prepared in Example 8 are observed with a transmission electron microscope. From the results, it was revealed that the nickel nanoparticles are larger in size than the nickel nanoparticles prepared in Example 1.

The particle sizes of the nickel nanoparticles prepared in Examples 1 to 8 were varied according to the conditions such as the reaction conditions and the presence of the reducing agent. Therefore, it was revealed that the nickel nanoparticles may be prepared in a simpler manner under the control of the sizes and shapes of nanosized nickel particles according to one exemplary embodiment of the present invention.

As described above, the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention may be useful to easily control the particle sizes and shapes of the nickel nanoparticles and mass-produce a high yield of the nickel nanoparticles that have a uniform size distribution with a size of 100 nm or less when the nickel nanoparticles are prepared using the method according to one exemplary embodiment of the present invention.

In accordance with the exemplary embodiment of the present invention, since the organic amine is used to prepare the nickel nanoparticles, the prepared nickel nanoparticles are coated with the organic amine. Therefore, the nickel nanoparticles have excellent dispersing property with respect to other organic solvents when the nickel nanoparticles will be used later. Therefore, the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention may be useful to simplify the preparation processes since an additional process is not required to disperse the nickel nanoparticles in other solvents.

Furthermore, the method for preparing nickel nanoparticles according to one exemplary embodiment of the present invention may be useful to more effectively prepare the nickel nanoparticles having a desired particle size since the sizes of the nickel nanoparticles may be controlled according to the concentration of the nickel precursor, the concentration and kinds of the reducing agents and/or the reaction temperature.

While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims. 

1. A method for preparing nickel nanoparticles, comprising: mixing a nickel precursor, organic amine and a reducing agent to prepare a mixture; and heating the mixture.
 2. The method of claim 1, wherein an organic solvent is further mixed with the mixture.
 3. The method of claim 1, wherein the nickel precursor is at least one selected from the group consisting of nickel chloride (NiCl₂), nickel sulfate (NiSO₄), nickel acetate (Ni(OCOCH₃)₂), nickel acetylacetonate (Ni(C₅H₇O₂)₂), halogenated nickel (NiX₂, wherein X is F, Br, or I), nickel carbonate (NiCO₃), nickel cyclohexanebutyrate ([C₆H₁₁(CH₂)₃CO₂]₂Ni), nickel nitrate (Ni(NO₃)₂), nickel oxalate (NiC₂O₄), nickel stearate (Ni(H₃C(CH₂)₁₆CO₂)₂), nickel octanoate ([CH₃(CH₂)₆CO₂]₂Ni) and hydrates thereof.
 4. The method of claim 1, wherein the organic amine is presented by C_(n)NH₂ (wherein, n is an integer of 4≦n≦30).
 5. The method of claim 1, wherein the organic amine comprises at least one selected from the group consisting of oleyl amine, dodecyl amine, lauryl amine, octyl amine, trioctyl amine, dioctyl amine and hexadecyl amine.
 6. The method of claim 1, wherein the reducing agent comprises at least one selected from the group consisting of sodium borohydride (NaBH₄), tetrabutylammonium borohydride ((CH₃CH₂CH₂CH₂)₄N(BH₄)), lithium aluminumhydride (LiAlH₄), sodium hydride (NaH), borane-dimethylamine complex((CH₃)₂NH.BH₃) and alkanediol (HO(CH₂)_(n)OH, wherein n is an integer of 5≦n≦30).
 7. The method of claim 2, wherein the organic solvent comprises at least one selected from the group consisting of an ether-based organic solvent (C_(n)OC_(n), wherein n is an integer of 4≦n≦30), a saturated hydrocarbon-based organic solvent (C_(n)H_(2n+2), wherein n is an integer of 7≦n≦30), an unsaturated hydrocarbon-based organic solvent (C_(n)H_(2n), wherein n is an integer of 7≦n≦30), and an organic acid-based organic solvent (C_(n)COOH, wherein n is an integer of 5≦n≦30).
 8. The method of claim 7, wherein the ether-based organic solvent is at least one selected from the group consisting of trioctylphosphine oxide, alkyl phosphine, octyl ether, benzyl ether, and phenyl ether.
 9. The method of claim 7, wherein the saturated hydrocarbon-based organic solvent is at least one selected from the group consisting of hexadecane, heptadecane and octadecane.
 10. The method of claim 7, wherein the unsaturated hydrocarbon-based organic solvent is at least one selected from the group consisting of octene, heptadecene and octadecene.
 11. The method of claim 7, wherein the organic acid-based organic solvent is at least one selected from the group consisting of oleic acid, lauric acid, stearic acid, mysteric acid and hexadecanoic acid.
 12. The method of claim 1, wherein the operation of heating the mixture is performed at a temperature of 50 to 450° C.
 13. The method of claim 1, wherein the operation of heating the mixture is performed for 1 minute to 8 hours.
 14. The method of claim 1, further comprising: separating the nickel nanoparticles from the heated mixture.
 15. The method of claim 14, wherein the operation of separating the nickel nanoparticles from the heated mixture comprises: adding ethanol or acetone to the heated mixture to precipitate the nickel nanoparticles. 